Device and method for electron transfer from a sample to an energy analyzer and electron spectrometer device

ABSTRACT

An electron imaging apparatus  100  is disclosed, which is configured for an electron transfer along an electron-optical axis OA of an electron  2  emitting sample  1  to an energy analyzer apparatus  200 , and comprises a sample-side first lens group  10 , an analyzer-side second lens group  30  and a deflector device  20 , configured to deflect the electrons  2  in an exit plane of the electron imaging apparatus  100  in a deflection direction perpendicular to the electron-optical axis OA. An electron spectrometer apparatus, an electron transfer method and an electron spectrometry method are also described.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application claims priority to DE 10 2019 107 327.8, filed Mar. 21,2019, the contents of which are incorporated herein by reference intheir entireties for all purpose

FIELD OF THE INVENTION

The invention relates to an electron imaging apparatus and an electrontransfer method for transferring electrons from a sample to an energyanalyzer apparatus, in particular for momentum- and energy-resolveddetection of electrons, such as e.g. photoelectrons. The inventionfurther relates to an electron spectrometer apparatus that is providedwith the electron imaging apparatus, and an electron spectrometrymethod. Applications of the invention lie in the electron spectroscopicanalysis of samples.

BACKGROUND OF THE INVENTION

In the present description, reference is made to the following prior artdocuments, which illustrate the technical background to the invention:

-   [1] B. Wannberg in “Nucl. Instrum. Meth. A” 601 (2009) 182;-   [2] EP 2 823 504 B1;-   [3] U.S. Pat. No. 9,997,346 B1;-   [4] EP 2 851 933 B1;-   [5] SE 539 849 C2;-   [6] DE 10 2005 045 622 B4;-   [7] DE 10 2013 005 173 B4;-   [8] EP 1 559 126 B9;-   [9] DE 10 2014 019408 B4; and-   [10] M. Patt et al. in “Review of Scientific Instruments” 85, 113704    (2014).

The use of transfer optics with electron-optical lenses is generallyknown for transferring and focusing electrons from a solid sample to anenergy analyzer, in particular for the measurement of photoelectrons andAuger electrons. Various types of energy analyzers and associatedtransfer optics are known. A group of methods, in which the electronsemitted from the sample are detected with angular resolution are knownas ARPES methods (ARPES: angular-resolved photo-electron spectroscopy)(see e.g. [1]). In the ARPES methods, the emission angle relative to areference axis, e.g. a surface normal of the sample surface, at whichelectrons are emitted from the sample, is of particular interest.Although high angle resolutions were achieved in the past, this was onlywithin relatively limited angle ranges. For example, a high resolutionof 0.1° in an angle range of approximately +/−7° and a reducedresolution of 0.5° in a range of approximately +/−15° has hitherto beenachievable.

Hemispherical analyzers assembled from two concentric hemisphericalelectrodes are typically used as energy analyzers in conventionalangular imaging using ARPES methods. Depending upon the voltage betweenthe hemispherical electrodes, only particles in a specific energy band(e.g. energy interval with a width of 10% of the pass energy of thehemispherical analyzer) are allowed to pass through the hemisphericalanalyzer. Imaging of the angular distribution of the electrons on anentry plane of the energy analyzer is provided for an energy- andangular-resolved measurement. An angular distribution image of electronsfrom the sample is generated in the entry plane of the energy analyzerin real space (in polar coordinates). Since a slit diaphragm is arrangedas an entrance slit in the entry plane of a hemispherical analyzer, saidhemispherical analyzer is able to determine the angular distributionalong a first angular coordinate (e.g. along the y-direction) along theextent of the entrance slit. In order to also determine the angulardistribution along a second coordinate (e.g. x) perpendicular to thefirst angular coordinate, an arrangement of two deflectors [2] or onedeflector [3] in the transfer lens sequentially scans (scanningmovement) the angular distribution image perpendicular to the extent ofthe entrance slit and to the electron-optical axis.

Conventional transfer optics for angular-resolved measurement ofelectrons have a major disadvantage in that, at the outlet from thetransfer optics and/or in the entry plane of the energy analyzer, theelectron beam enters the energy analyzer with a marked divergencerelative to the electron-optical axis, e.g. z-axis, of typically >10°.The divergence is illustrated in FIG. 7 (prior art, cited from [2] usingthe example of the trajectory of a few electron partial beam from asample 1′ with different emission angles in a y-z-plane (see also [1]and [3]). It causes imaging errors in the energy analyzer and requires asignificant amount of postprocessing effort to calculate energy- andangular distributions of the electrons. Furthermore, conventionaltransfer optics do not have a sufficiently well focused and localizedposition space image of the sample source spot, from which the electronsare emitted (see e.g. [1]).

The divergence is particularly disadvantageous in the acquisition of theangular distribution along the second angular coordinate, wherein thesecond angular coordinate is sequentially scanned with the aid of one ortwo deflectors 20A′, 20B′ in the transfer optics (see [1], [2], [3], [4]and [5]). With the scanning motion of the divergent beam perpendicularto the extent of the entrance slit, combination errors are created fromthe beam divergence in the entry plane in combination with aberrationsinduced by the deflection of the beam by the deflectors.

The beam divergence in the entry plane is also disadvantageous inspin-resolved electron spectroscopy, in which an imaging spin-filterwith a spin-filter crystal is arranged at the outlet of the energyanalyzer (see [6]). The beam divergence along the entrance slit in theentry plane causes a beam divergence in the exit plane in the form of amarked astigmatism, which makes a well-focused image of the exiting beamon the spin-filter crystal impossible.

In summary, the high angular divergence of the electron beam at the endof conventional transfer optics has considerable disadvantages forangular-resolved spectroscopy of electrons using energy analyzers, inparticular using hemispherical analyzers. These disadvantages limit thepotentially detectable angular intervals and the effect is even greaterwhen the scanning motion is used to capture the second angularcoordinate, and when an imaging spin-filter is used.

Documents [7], [8], [9] and [10] describe additional transfer opticsintended for momentum microscopy and containing telescopic beam paths,in which there are Gaussian images (real space images) and momentumdistribution images (momentum space images). However, these transferoptics are not designed for the scanning motion of a momentumdistribution image perpendicular to the extent of an entrance slit of anenergy analyzer. Furthermore, all these techniques are based on the useof cathode lenses, wherein the electrons are accelerated into thetransfer optics by means of a strong electrostatic extractor field.However, the strong extractor field is disturbing in the examination ofnon-planar surfaces (such as can occur e.g. after splitting of a samplein the UHV), microstructures (such as e.g. semiconductor components) ormicrocrystals with a three-dimensional structure (common test objects ofnew quantum materials).

The objective of the invention is to improve an electron imagingapparatus, an electron transfer method, an electron spectrometerapparatus and/or an electron spectrometry method in such a way as toavoid disadvantages of conventional techniques. The objective isparticularly to minimize or even eliminate divergence-induced imagingerrors, achieve a better angular resolution, achieve the detection oflarger angular intervals and/or simplify spin-resolved electronspectroscopy.

These objectives are correspondingly achieved by an electron imagingapparatus, an electron transfer method, an electron spectrometerapparatus and an electron spectrometry method of the invention.

BRIEF SUMMARY OF THE INVENTION

According to a first general aspect of the invention, the aboveobjective is achieved by an electron imaging apparatus that isconfigured for an electron transfer along an electron-optical axis froman electron-emitting sample to an energy analyzer apparatus. Theelectron transfer comprises a transfer and imaging of the emittedelectrons (also referred to as electron bundle or electron beam) from anentry plane to an exit plane of the electron imaging apparatus, whereinthe entry plane is provided on the surface of the sample facing towardsthe electron imaging apparatus and the exit plane is provided on therear side of the electron imaging apparatus facing towards the energyanalyzer apparatus and are consequently also referred to as sample-sideand analyzer-side entry plane and exit plane respectively. Theelectron-optical axis is preferably a continuous straight axis extendingperpendicular to the entry and exit planes but can alternatively have anangled course that is straight in sections.

The electron imaging apparatus comprises a sample-side first lens group,an analyzer-side second lens group and a deflector device configured forelectrical and/or magnetic deflection of the electrons in the exit planeof the electron imaging apparatus in a deflection directionperpendicular to the electron-optical axis. Each of the first and secondlens group comprises at least two electron-optical lenses. The first andthe second lens group and the deflector device are each connected to acontrol circuit to provide operating voltages, wherein the controlcircuits can be separate components or connected to a common controldevice.

According to the invention, the first lens group is configured to form afirst reciprocal plane inside the first lens group and a first Gaussianplane between the first and the second lens group and to generate afirst momentum distribution image of a momentum distribution ofelectrons from the sample in the first reciprocal plane and to generatea first Gaussian image of the sample, in particular of the illuminatedsample source spot, in the first Gaussian plane. A reciprocal plane isan imaging plane in which there is a reciprocal image (also known as amomentum image or Fourier image). Accordingly, the first reciprocalimage, referred to here as a momentum distribution image, is focused inthe first reciprocal plane. The momentum distribution image is an imageof the momentum distribution of the electrons, wherein the transversemomentum of the electrons increases with increasing distance of thepartial beams of the momentum distribution image from theelectron-optical axis. The control circuit of the first lens group isconfigured to apply suitably adapted control voltages to the first lensgroup for electron-optical imaging of the first reciprocal plane and thefirst Gaussian plane.

Moreover, according to the invention, the second lens group isconfigured to form a second reciprocal plane on the analyzer side of thesecond lens group and to generate a second momentum distribution imageof the momentum distribution of the electrons from the sample in thesecond reciprocal plane. The control circuit of the second lens group isconfigured to apply suitably adapted control voltages to the second lensgroup for electron-optical providing of the second reciprocal plane.

Moreover, according to the invention, the first lens group is configuredto generate the first Gaussian image with such a small dimension (extentof image perpendicular to the optical axis) that the second momentumdistribution image generated by the second lens group is a parallelimage.

The deflector device preferably acts in a Gaussian plane, e.g. in thefirst Gaussian plane or in another Gaussian plane (see below), of theelectron imaging apparatus, so that the deflection of the electronsadvantageously effects a parallel displacement of the partial beams,which form the parallel image in the second reciprocal plane, withoutchanging the angle between the partial beams perpendicular to theoptical axis.

The term “parallel image” (or: substantially parallel angular image ormomentum distribution image with substantially parallel partial beams)hereinafter refers to a momentum distribution image, the partial beamsof which run parallel to the electron-optical axis on passing throughthe second reciprocal plane and/or have such a minimal divergence thataberrations (in particular divergence-induced imaging errors) in theenergy analyzer apparatus are negligibly small for the desiredenergy-resolved measurement of electrons, in particular do not cause anysignificant impairment of the spectroscopic properties (energyresolution, momentum resolution) of the energy analyzer apparatus. Theparallel image is composed of partial beams that are alignedperpendicular or virtually perpendicular to the second reciprocal plane.The partial beams enter the energy analyzer apparatus as parallelbundle. The substantially perpendicular alignment of the partial beamsextends over an angular range of the electrons emitted from the samplethat is to be detected in the subsequent energy analysis in the energyanalyzer apparatus.

The control circuit of the first lens group is configured to generatethe control voltages of the first lens group such that the firstGaussian image is of the desired size. The geometry of theelectron-optical lenses of the first lens group is preferably configuredsuch as to produce the smallest possible Gaussian image of the sourcespot of the electrons on the sample, in the first Gaussian plane.Optimization of the geometry of the electron-optical lenses takes placee.g. by numerical simulations.

The parallel image (parallel momentum distribution image in the exitplane) preferably has angular deviations (divergences) of its partialbeams that are smaller than 0.4°, particularly preferably smaller than0.2°. In simulations, the angular deviations have shown themselves to besmall enough (see below, FIG. 2) not to compromise the subsequentimaging in the energy analyzer apparatus.

According to a further advantageous embodiment of the invention, thefirst lens group is configured to generate the first Gaussian image withan extent perpendicular to the electron-optical axis of less than 1 mm,in particular less than 0.5 mm. For these size ranges, the parallelmomentum distribution image is advantageously achieved with sufficientlysmall angular deviations in the typical measurement tasks that occur inpractice and in the typical configurations of electron optics.

The control circuit of the deflector device is configured for a scanningdeflection of the momentum distribution image in the second reciprocalplane perpendicular to the electron-optical axis. Accordingly, theelectrons in the exit plane of the electron imaging apparatus aresubjected to a scanning motion in a direction that deviates from theextent of an entrance slit (also referred to as slit or slit diaphragm)of the energy analyzer apparatus, preferably perpendicular to theentrance slit. The configuration of the first and second lens groups andthe deflector device or correspondingly related control circuits isadjusted on the basis of calculations using electron-optical imagingequations that are known per se and the geometric size of the electronimaging apparatus.

According to a second general aspect of the invention, the aboveobjective is achieved by an electron spectrometer apparatus comprising asample-holder to hold a sample, the electron imaging apparatus accordingto the first general aspect of the invention or an embodiment thereofand an energy analyzer apparatus. According to the invention, theelectron imaging apparatus is configured for the transfer and imaging ofelectrons emitted from the sample along the electron-optical axis to theenergy analyzer apparatus and for the scanning motion of the momentumdistribution image perpendicular to the electron-optical axis and in adeflection direction, which deviates from the extent of an entrance slitof the energy analyzer apparatus, preferably in a deflection directionperpendicular to the extent of the entrance slit. The electron imagingapparatus is configured such that a momentum distribution image of theelectrons emitted from the sample source spot is generated as a parallelimage in the entry plane of the energy analyzer apparatus.

The energy analyzer apparatus is generally an electron-optical imagingdevice configured for an angular- or momentum-resolved detection of theangular- or momentum distribution image along at least one directionperpendicular to the electron-optical axis. The energy analyzerapparatus preferably comprises a hemispherical analyzer (hemisphericalelectron energy analyzer). By using the hemispherical analyzer withentrance slit, it is possible to determine the angular distribution in adirection along the entrance slit and in a second, deviating directionby the scanning motion of the momentum distribution image by means ofthe deflector device. As an alternative to using a hemisphericalanalyzer, the energy analyzer apparatus can comprise other types ofanalyzers such as e.g. a cylinder analyzer or a 127° analyzer.

According to a third general aspect of the invention, the aboveobjective is achieved by an electron transfer method, wherein electronsare transferred by an electron imaging apparatus from a sample along anelectron-optical axis to an energy analyzer apparatus.

The sample preferably comprises a solid sample, the surface of which isexposed for an irradiation with excitation light and for the emissiontowards the electron imaging apparatus. The electrons emitted from thesource spot (incident surface of the excitation light beam) aresequentially transferred through a sample-side first lens group, adeflector device and an analyzer-side second lens group, wherein theelectrons are deflected by the deflector device in an exit plane of theelectron imaging apparatus in a deflection direction runningperpendicular to the electron-optical axis and deviating from the extentof the entrance slit of the energy analyzer apparatus, preferably isaligned perpendicular to the extent of the entrance slit. According tothe invention, the first lens group forms a first reciprocal planewithin the first lens group and a first Gaussian plane between the firstand the second lens groups, generating a first momentum distributionimage of a momentum distribution of electrons from the sample in thefirst reciprocal plane and a first Gaussian image of the source spot inthe first Gaussian plane. Moreover, the second lens group forms a secondreciprocal plane on the analyzer side of the second lens group and asecond momentum distribution image of the momentum distribution of theelectrons from the sample in the second reciprocal plane. The first lensgroup generates the first Gaussian image with such a small dimensionthat the second momentum distribution image generated by the second lensgroup is a parallel image. The electron transfer method is preferablyexecuted by the electron imaging apparatus according to the firstgeneral aspect of the invention or embodiments thereof.

According to a fourth general aspect of the invention, the aboveobjective is achieved by an electron spectrometry method (method fordetecting the energy and momentum distribution of electrons that havebeen emitted from a sample) comprising an irradiation of a sample andemission of electrons from the sample, a transfer of the emittedelectrons by an electron transfer method according to the third generalaspect of the invention or an embodiment thereof to an energy analyzerapparatus and a momentum- and energy-resolved detection of the electronsby the energy analyzer apparatus. The electron spectrometry method ispreferably executed by the electron imaging apparatus according to thesecond general aspect of the invention or an embodiment thereof.

The present invention advantageously provides apparatuses and methodsfor substantially improving the beam transfer for electrons emitted froma solid sample in preferably field-free environment and analyzed in theenergy analyzer apparatus in terms of their energy- and momentumdistribution. Imaging errors that occur in conventional electron imagingapparatuses with the generation of angular images in the real space areavoided by the generation of a momentum distribution image as a parallelimage. Since a suitably small Gaussian image of the area illuminated bythe excitation source (sample source spot) is positioned in a frontfocal plane of the second lens group, so that the momentum distributionimage that is focused in the plane of the entrance slit of the energyanalyzer apparatus, is created from parallel partial beams in a rearreciprocal plane of this lens group, all electrons enter the energyanalyzer apparatus substantially parallel to the electron-optical axis.As a result, the angular deviations at the entrance of the energyanalyzer apparatus are reduced by 1 to 2 orders of magnitude relative toconventional methods. The first Gaussian image is generated by means ofthe first lens group, which is preferably aberration-minimized by meansof suitable geometry.

An important advantage of the invention therefore lies in thepossibility of capturing substantially larger angular ranges of theelectrons emitted from the sample. Since the relationship between thetransverse momentum k₁ and image angle α scales with the root of theenergy:

k₁˜sin α✓E_(kin), at lower kinetic energies (e.g. equal to or less than5 electron volts, such as in laser excitation, for example), angularintervals of more than +/−30° are possible and this considerablyenhances the efficiency of the energy analyzer. Particular advantages ofthe invention also result from scanning of the second momentumcoordinate to measure a two-dimensional momentum distribution and fromthe use of an imaging spin-filter. By positioning the deflector devicein the first Gaussian image and focus of the second lens group, it ispossible to achieve a parallel displacement of the momentum distributionimage in the entry plane of the energy analyzer apparatus preferablywith a single deflector unit, wherein the parallelism of the beam ispreserved.

The aberrations of the combination of lens groups and deflector devicecan be advantageously reduced to the theoretically possible minimum, inthat the image reciprocal to the desired momentum distribution image(i.e. the first Gaussian image of the source spot on the sample) is sosmall that the coding of the momentum distribution image is defined withextreme precision as a trajectory in this small Gaussian image. Thischaracteristic distinguishes the present invention from all conventionalbeam transfer systems, in particular those according to [1, 2, 3, 4 and5].

The inventors have found that, using the technique according to theinvention, a concept realized in electron microscopy, in particular thecreation of predetermined, very well focused real space images andreciprocal images in predetermined imaging planes using the opticalimage transition theorem, not only serves to reduce aberrations in theelectron-optical system in electron spectroscopy but offers anadditional substantial advantage: since the first Gaussian image can bedisplaced along the electron-optical axis by means of the first lensgroup, the tilt angle of the individual partial beams of the momentumdistribution image can be minimized in the entry plane of the energyanalyzer apparatus, without losing the beam position. This allows thedeflection technique to be used for scanning the second momentumdirection, in particular perpendicular to the entrance slit, even forhigh retardation conditions (ratio of the initial kinetic energy of theelectrons on exit from the sample to the kinetic energy on entry intothe energy analyzer apparatus). The preferred embodiment of the secondlens group in the form of a zoom lens (lens that is configured foradjustment of an image magnification and preferably comprises five ormore lens elements) makes it possible, in particular, to vary the imagesize and electron energy of the momentum image at the outlet from theelectron imaging apparatus within broad limits and so adapt the momentumimage size and energy to the desired conditions for the energy analyzer(energy resolution, momentum resolution).

The creation of the parallel image in the entry plane of the energyanalyzer apparatus also advantageously simplifies a spin-resolvedelectron spectroscopy, since image errors on a spin-filter crystal atthe outlet of the energy analyzer are reduced or avoided altogether.

A further particularly important advantage of the invention is thatdeflection of the momentum distribution image is simplified. Thus, in apreferred embodiment of the invention, it is provided that the deflectordevice only acts in a single plane perpendicular to the electron-opticalaxis. On transfer of the electrons along the electron-optical axis, thedeflector device acts one single time, when the electrons pass throughthe space between the first lens group and the second lens group. Thisadvantageously avoids conventional, complex double deflectors, e.g.according to [2], without having to accept imaging errors. Particularlypreferably, the deflection plane of the deflector device and the firstGaussian plane (or another Gaussian plane, see below) coincide, so thatan exact parallel displacement of the momentum distribution image in thesecond reciprocal plane is facilitated by actuating the deflectordevice. This completely avoids any additional beam tilting, such as isdescribed in [3] and [5] for example.

Deflection in one single plane has the further advantage that thedeflector device can be of particularly simple construction, e.g. withone single pair of electrically and/or magnetically acting deflectorelements, a quadrupole arrangement or an octupole arrangement ofdeflector elements. In the case of electrical deflection, the deflectorelements comprise deflecting electrodes and, in the case of magneticdeflection, the deflector elements comprise deflecting coils. Theoctupole arrangement advantageously allows a rotation of the deflectiondirection around the electron-optical axis, so that the deflector devicecan additionally rotate the direction of displacement of the momentumdistribution image relative to the alignment of the entrance slit of theenergy analyzer apparatus and, in this way, compensate for anyundesirable image rotations due to magnetic stray fields, for example.

According to a further advantageous embodiment of the electron imagingapparatus (hereinafter also referred to as the first embodiment), thesecond reciprocal plane is at the same time the exit plane of theelectron imaging apparatus, wherein a slit diaphragm, which forms theentry slit of the energy analyzer apparatus, is arranged in the secondreciprocal plane. This embodiment offers the advantages of an adjustmentof the parallel image directly on entry into the energy analyzerapparatus and a relatively compact construction.

According to alternative embodiments of the electron imaging apparatus,it has at least one third lens group, which is arranged on the analyzerside of the second lens group, i.e. in the overall arrangement of theelectron spectrometer apparatus, between the second lens group and theenergy analyzer apparatus. The at least one third lens group forms thesecond Gaussian plane within the at least one third lens group and athird reciprocal plane on the analyzer side of the at least one thirdlens group, so that a second Gaussian image of the sample in the secondGaussian plane is created by the at least one third lens group and athird momentum distribution image of the momentum distribution of theelectrons from the sample is created in the third reciprocal plane. Thethird reciprocal plane is the exit plane of the electron imagingapparatus and the entry plane of the energy analyzer apparatus and thethird momentum distribution image generated by the at least one thirdlens group is a parallel image. Even in the case of these alternativeembodiments of the invention, the scanning motion of the momentumdistribution image, which effects a corresponding deflection of themomentum distribution image in the third reciprocal plane, is provided.The at least one third lens group is provided with a control circuitconfigured to apply suitably adapted control voltages to the at leastone third lens group for electron-optical forming of the thirdreciprocal plane and the second Gaussian plane. The provision of the atleast one third lens group has the particular advantage that the beamenergy in the parallel momentum distribution image can be varied in theentry plane of the energy analyzer apparatus within a range of e.g. twoorders of magnitude, so allowing optimum resolution and transmissionsettings of the energy analyzer apparatus.

A slit diaphragm can be arranged in the second reciprocal plane in frontof the third lens group to form the entrance slit of the energy analyzerapparatus (hereinafter referred to as the second embodiment). In thiscase, the slit diaphragm is not positioned directly in the entry planeof the energy analyzer apparatus but in the second reciprocal plane,which is conjugated to the entry plane, in front of the third lensgroup, where the parallel momentum distribution image is generated, sothat a real image of the slit diaphragm overlaid by the parallelmomentum distribution image is generated in the entry plane of theanalyzer. In this arrangement, the electrons can advantageously passthrough the entrance slit with much greater energy, which improvessubsequent imaging in the energy analyzer apparatus. Since the slitdiaphragm is arranged in the second reciprocal plane, the slit diaphragmcan be imaged as a real image in the entry plane of the energy analyzerapparatus, so that the effective slit diaphragm width and energy of theelectrons in the entry plane can be electron-optically varied, while thephysical slit diaphragm width being fixed.

Alternatively, the slit diaphragm forming the entrance slit of theenergy analyzer apparatus can be arranged in the third reciprocal planeand forms the entrance slit of the energy analyzer apparatus, wherein,in this case, no slit diaphragm is arranged in the second reciprocalplane (hereinafter also referred to as the third embodiment). The thirdembodiment offers the particular advantage that an energy analyzer ofconventional design, i.e. with integrated entrance slit, can be used.

According to further modified embodiments of the invention, a fourthlens group or additional lens groups, which are each constructed likethe third lens group and provide additional Gaussian planes andreciprocal planes, can be provided.

Advantageously, there are various options for positioning the deflectordevice. Particularly in the first, second and third embodiment of theinvention, the deflector device can be disposed between the first andthe second lens group and in the first Gaussian plane. Alternatively, inthe second and third embodiment of the invention, the deflector devicecan be disposed at the third lens group and in the second Gaussianplane. According to further alternatives, the deflector device can bedisposed in a further Gaussian plane in the fourth lens group or anadditional lens group.

A further important difference relative to electron-optical concepts ofelectron momentum microscopy is achieved by an advantageous embodimentof the invention, wherein the sample-side foremost electron-optical lenselement of the first lens group is configured to have the same potentiallike the sample. Advantageously, this keeps the area around the samplefield-free so that it is also possible to examine samples with highlythree-dimensional structures or microcrystals without any significantfield distortion due to the three-dimensional shape, which wouldcompromise momentum resolution.

For displacement of the momentum distribution image in the secondreciprocal plane the deflector device particularly preferably acts inthe first Gaussian plane. Arrangement in the first Gaussian plane meansthat the deflector device generates varying deflector fields, which actin a narrow range symmetrically around the first Gaussian plane. Thisembodiment has the advantages of a particularly effective deflection ofthe entire electron beam, wherein the parallelism of the momentum imageat the outlet of the imaging device is preserved. Alternatively, whenproviding the third lens group, the single deflector device can bearranged in the second Gaussian plane and act on the electrons in thesecond Gaussian image, wherein the parallelism of the momentum image islikewise preserved at the outlet of the imaging device on deflection.

According to a further advantageous embodiment of the invention, it isprovided that the analyzer-side rearmost electron-optical lens of thesecond lens group forms a field-free space in the region of the secondreciprocal plane. Alternatively or additionally, it is provided in thesecond or third embodiment that the third lens group has boundaryelements at its ends, said boundary elements being configured to formfield-free spaces adjacent to both sides of the third lens group. Thisembodiment prevents any electrostatic feed-over of fields leading to adeflection of the electron pathways and hence to a distortion of themomentum image.

BRIEF DESCRIPTION OF THE DRAWINGS

Further details and advantages of the invention are described below withreference to the attached drawings. The drawings show in:

FIG. 1: schematic illustrations of the first embodiment of the electronimaging apparatus according to the invention;

FIG. 2: further illustrations of the generation of a Gaussian image onexcitation of a sample by various excitation sources;

FIG. 3: a schematic illustration of the second embodiment of theelectron imaging apparatus according to the invention;

FIG. 4: a schematic illustration of the third embodiment of the electronimaging apparatus according to the invention;

FIG. 5: schematic illustrations of different variants of a deflectordevice;

FIG. 6: a schematic illustration of an embodiment of the electronspectrometer apparatus according to the invention; and

FIG. 7: a diagram of beam trajectories of a conventional transferoptical system (cited from [2]).

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS

The invention is described below with exemplary reference to an electronimaging apparatus in combination with a hemispherical analyzer. Theinvention is not limited to the use of the hemispherical analyzer but isalso executable with other types of energy analyzer apparatuses. Detailsof the excitation of a sample and recording of energy distributions ofthe electrons emitted from a sample with the hemispherical analyzer arenot described, since these are known per se from conventionaltechniques. The illustrations of the electron-optical components in thedrawings are schematic illustrations. Details such as e.g. thearrangement of the electron-optical components in an evacuated space orthe formation of electron-optical lenses from spaced lens elements, arenot shown. In general, e.g. electron-optical lenses and the associatedcontrol circuits can be the same as those essentially known fromconventional transfer optics.

The Figures are described with reference to the relevant spatialdirections shown in FIG. 1, which, in the position-space, comprise the zdirection along the electron-optical axis and, perpendicular to this,the x and y directions, wherein a slit diaphragm 201 in the entry planeof the energy analyzer apparatus 200 (see also FIG. 6) extends in the ydirection. Accordingly, the direction of the slit diaphragm defines thedirection of the momentum coordinate k_(y) and, perpendicular to it, themomentum coordinate k_(x).

FIG. 1 shows the first embodiment of the electron imaging apparatus 100according to the invention in the form of an electron-optical system forthe transfer and transverse displacement of a parallel momentumdistribution image based on an electron-optical column with multiplelens groups 10, 30 and a field-free drift path 22, in which is disposeda deflector device 20. FIG. 1A shows the construction of the electronimaging apparatus 100, FIG. 1B shows the simulated beam path for arealistic optics with the deflector device 20 switched off, and FIG. 1Cshows the simulated beam path for the same lens settings as in FIG. 1B,but with the deflector device 20 switched on by applying suitablevoltages to the deflector electrodes 21. In FIGS. 1B, C, and in allrepresentations of electron trajectories in FIGS. 2, 3 and 4, the radialcoordinate is radially enlarged to show the details of the individualbeams more clearly. The lens groups 10, 30 and the deflector device 20are connected to a control device 50, comprising control circuits forenergizing the electron-optical lenses or electrodes of the components10, 20 and 30. The electron imaging apparatus 100 is arranged on asample-holder 101 with a distance of e.g. 15 mm between the surface of asample 1 and a front cap electrode 11 of the first lens group 10, suchthat the sample 1 is located in the object plane of the first lens group10. The length of the slit diaphragm 201 is e.g. 20 mm to 40 mm, and itswidth is e.g. 50 μm to 2 mm.

Specifically, the first lens group 10 according to FIG. 1A comprises afront cap electrode 11, a focusing electrode 12 and adapter lenses 13.In order to create a field-free space between the sample 1 and the firstlens group 10, the sample-holder 101 can be electrically connected tothe front cap electrode 11 so that both components are at the sameelectrical potential. The second lens group 30 comprises multiple lenselements 31, which preferably form a zoom lens, and a boundary element32 to create a field-free space between the rearmost lens element andthe slit diaphragm 201 of the energy analyzer apparatus 200. To thisend, the boundary element 32 and the slit diaphragm 201 are electricallyconnected to each other.

An e.g. cylindrical element is provided to form the field-three driftpath 22 between the first lens group 10 and the second lens group 30,the length of said cylindrical element being such that any feed-over ofthe adjacent lens groups is reduced and there is no longer anysignificant electrical field, which could deflect the electrontrajectories in the region of the Gaussian image, at the site of thedeflector device when it is switched off.

The deflector device 20 comprises e.g. an octupole arrangement ofdeflecting electrodes 21 or alternatively another electrode arrangement(see FIG. 5). The deflector device 20 is preferably a single deflector,i.e. the electrons are deflected once only as they pass through thefirst Gaussian plane on their way along the electron-optical axis OAbetween the sample and the second reciprocal plane.

When the sample 1 is excited by light (see also FIG. 2), an ensemble ofelectrons 2 is emitted from the sample (FIG. 1B), wherein the electrons2 are imaged by the first lens group 10 up to a predetermined emissionangle of e.g. +/−15° along the electron-optical axis OA. According toFIG. 1B, which shows the electron imaging apparatus 100 in the x-z planewith the slit diaphragm 201 of the energy analyzer apparatus 200perpendicular to the drawing plane, the first lens group 10 generates areciprocal image of the electrons 2 emitting sample source spot in thefirst reciprocal plane RP1 and a first Gaussian image of the samplesource spot in the first Gaussian plane GP1, which is a posterior focalplane of the first lens group 10. A second reciprocal image (momentumdistribution image with parallel partial beams) is imaged with thesecond lens group 30 into the entrance slit of the energy analyzerapparatus 200. It is thereby guaranteed that all electrons enter theenergy analyzer apparatus 200 as parallel partial beams with very smallangular deviations. The first lens group 10 is controlled such that theintersection of the first Gaussian plane GP1 with the electron-opticalaxis OA is positioned in the deflection plane of the deflector device20, in particular in the center of the deflector device 20. Centering ofthe Gaussian image on the deflector device 20 allows precise paralleldisplacement of the momentum distribution image with parallel partialbeams generated by the second lens group 30 in the second reciprocalplane RP2. By applying suitable voltages to the deflector device 20,parallel displacement of the momentum distribution image in the secondreciprocal plane RP2 is possible without additional tilting of the beam,as shown in FIG. 1C.

FIG. 1C shows the effect of the deflector device 20 acting in the firstGaussian plane GP1 to provide optimized lens geometry and practicalmeasuring conditions (+/−15° angular interval, kinetic energy on thesample 16 eV, which suits a conventional vacuum ultraviolet laboratorylight source). In the illustrated example, at the power of the deflectordevice 20 set by the voltages on the deflecting electrodes 21, theanalyzer-side momentum distribution image in the second reciprocal planeRP2 is displaced in parallel by a momentum radius R. Diameter 2R andparallelism of the momentum distribution image are preserved on paralleldisplacement. Thus, by continuous variation of the deflecting force ofthe deflector device 20 by means of a control circuit (control device50), the two-dimensional momentum image can be fully scanned.

FIG. 2 shows the beam trajectory of the electrons 2 as in FIG. 1B withdifferent excitation sources (not shown) to generate the electrons 2,wherein FIG. 2A shows excitation by the light beam of a synchrotronradiation source or a laser, FIG. 2B shows excitation by a focusedvacuum UV excitation source and FIG. 2C shows excitation by an unfocusedvacuum UV excitation source.

The angle θ designates the emission angle of the electrons from thesample relative to the electron-optical axis OA and the angle α the(half) opening angle of a beam bundle, which corresponds to a specificemission angle, of the momentum distribution image in the secondreciprocal plane RP2. The amount of this opening angle α of the partialbeams (virtually invisible in the detail in FIG. 2A but clearly visiblein the detail in FIG. 2C) is determined by the size of the electronsource area on the sample surface and hence by the cross-section of theexciting light beam and its angle of incidence onto the sample.

Realistic calculations were carried out for a practical embodiment ofthe electron imaging apparatus 100 according to the invention using atrajectory simulation program (SIMION 8.0), three of these being shownas examples in FIG. 2. The distance along the axial coordinate z betweenthe surface of the sample 1 and the momentum distribution image in thesecond reciprocal plane RP2 is 462 mm. The electron-optical lenses areaberration-minimized and, in this case, a reduced image of the sourcearea on the sample was set in the Gaussian plane GP1 with amagnification factor M=0.6. Other magnifications, even those with M>1,are likewise possible with the invention.

The simulations for an angular acceptance range of 0=15° deliver thefollowing parameters: in all three cases, the momentum distributionimage in the second reciprocal plane RP2 has a radius R of 4.3 mm. Thetilts of the central rays of the beam (=deviation from the parallelbeam) are all <0.09° and, as would be expected, are not dependent uponthe size of the source area.

For excitation with synchrotron radiation or laser sources (FIG. 2A), 50μm was assumed as a typical size for the light beam on the sample. Dueto the large angular range of 0=15°, the first Gaussian image in thefirst Gaussian plane GP1 is widened by spherical aberration of the firstlens group 10 and has a radius of r˜40 μm. The partial beams in planeRP2 have an opening angle of α≤0.16°. This value is nearly two orders ofmagnitude smaller than the corresponding values of the angulardivergence in the conventional transfer lens systems, as is shown inFIG. 7, for example.

Focused vacuum UV light sources (FIG. 2B) have typical light spot sizesof 200 μm. In this case, the first Gaussian image in the first Gaussianplane GP1 has a radius of 80 μm, and the opening angle of the beam, i.e.the resulting angular divergence in the momentum image in plane RP2 isα≤0.2°.

For unfocused vacuum UV laboratory sources (FIG. 2C), an exciting lightbeam has a typical diameter of e.g. 0.5 mm, which, in the first Gaussianplane GP1, produces a first Gaussian image with radius r=155 μm. Here,the opening angle of the beam in the plane RP2 is α˜0.35°. The detail inFIG. 2C is greatly enlarged radially to make the small angle clear. Themomentum distribution image in the second reciprocal plane RP2 issharply focused in this case as well, and the electrons enter the energyanalyzer apparatus 200 as parallel beams with minimal divergence.

FIG. 3 shows a preferred variant of the second embodiment of theelectron imaging apparatus 100 according to the invention, wherein thecomponents 10, 20 and 30 and their parts are provided, as in FIG. 1.Unlike in the first embodiment, a third lens group 40 is additionallyprovided and this generates a second Gaussian image on the analyzer sidebehind the slit diaphragm 201 in the second Gaussian plane GP2 and athird momentum distribution image with parallel partial beams in a thirdreciprocal plane RP3. The third reciprocal plane RP3 is the entry planeof the energy analyzer apparatus 200, so that the electrons 2 enter theenergy analyzer apparatus 200 as a parallel bundle. The third lens group40 comprises electron-optical lens elements 42, which form a zoom lens.The lens elements 42 and the associated control circuit (not shown) areconfigured such that the electron energy and the lateral magnificationof the momentum distribution image in the reciprocal plane RP3 arevariable, in order to optimize the energy- and/or momentum resolution ofthe energy analyzer apparatus 200. At both ends of the zoom lenscomprising the lens elements 42, there are boundary elements 41 and 43arranged to create field-free spaces at the ends of the zoom lens andminimize any feed-over of electrical fields from the region of the slitdiaphragm 201 and the entry of the energy analyzer apparatus 200 in thethird reciprocal plane RP3. In this embodiment, there is an image of theslit diaphragm 201 in the entry plane of the energy analyzer apparatus200, but no physical slit, i.e. no physical slit diaphragm.

FIG. 4 shows a preferred variant of the third embodiment of the electronimaging apparatus 100 according to the invention, wherein the third lensgroup 40 with the lens elements 42 and the boundary elements 41, 43 isalso provided. However, with the third embodiment, the slit diaphragm201 is arranged in the third reciprocal plane RP3 in the entry of theenergy analyzer apparatus 200. In this case as well, the third lensgroup 40 comprises lens elements 42 forming a zoom lens, which allowsvarying the electron energy and the lateral magnification of themomentum distribution image in the third reciprocal plane RP3 in broadranges, in order to optimize energy- and momentum resolution of theenergy analyzer apparatus 200. In this case, a conventional energyanalyzer with integral entrance slit can be used.

FIG. 5 shows variants of the deflector device 20, which can be providedin various embodiments. Thus, the deflector device 20 according to FIG.5A comprises at least two electrodes 21 (termed parallel platedeflector) or, according to FIG. 5B, four electrodes 21 in one plane(termed x-y deflector) or, according to FIG. 5C, eight electrodes 21 inone plane (termed octupole arrangement or octupole deflector). Theembodiment according to FIG. 5C is particularly advantageous, sincedeflection can take place in any arbitrary plane by means of suitablevoltages on the eight electrodes 21, in order to align the deflectionplane exactly along the x direction, that is to say exactlyperpendicular to the direction of the entrance slit 201, as illustratedin FIGS. 1B and 1C. Moreover, the octupole arrangement makes it possibleto correct undesirable image rotations due to longitudinal magneticstray fields that might occur during the scanning process.

FIG. 6 is a diagram of an embodiment of the electron spectrometerapparatus 300, comprising the electron imaging apparatus 100, e.g.according to any of FIG. 1, 3 or 4, and the energy analyzer apparatus200 in the form of a hemispherical analyzer with an electron detector202. On implementation of the electron spectrometry method, theelectrons emitted from the sample 1 on the sample-holder 101 aretransferred by the electron imaging apparatus 100 along theelectron-optical axis OA to the energy analyzer apparatus 200. Anenergy- and momentum distribution of the electrons is recorded bycapturing momentum distribution images of the electrons along a firstmomentum coordinate along the slit diaphragm 201 and via the stepwisedisplacement of the momentum distribution image perpendicular to theslit diaphragm 201, so that the full two-dimensional momentumdistribution can be recorded.

The features of the invention disclosed in the above description, thedrawings and the claims are important for the realization of theinvention in its various embodiments both individually or in combinationor sub-combination.

LIST OF REFERENCE NUMERALS

-   100 Electron imaging apparatus-   101 Sample-holder-   200 Energy analyzer apparatus-   201 Slit diaphragm-   202 Electron detector-   300 Electron spectrometer apparatus-   1 Sample-   2 Electrons-   10 First lens group-   11 Front cap electrode-   12 Focusing electrode-   13 Adapting lens-   20 Deflector device-   21 Deflector element-   22 Drift tube-   30 Second lens group-   31 Lens elements-   32 Boundary element-   40 Third lens group-   41 Boundary element-   42 Zoom lens-   43 Boundary element-   50 Control device-   RP1 First reciprocal plane (momentum image plane)-   GP1 First Gaussian plane (plane of real space image)-   RP2 Second reciprocal plane-   GP2 Second Gaussian plane-   RP3 Third reciprocal plane-   OA Optical axis-   θ Emission angle relative to OA-   α Opening angle of beam bundles-   R Radius of the momentum distribution image-   r Radius of the Gaussian image-   x, y, z Directional coordinates-   k_(x), k_(y), k_(z) Momentum coordinates

What is claimed is:
 1. An electron imaging apparatus that is configuredfor an electron transfer along an electron-optical axis from a sampleemitting electrons to an energy analyzer apparatus and comprises a firstlens group on a sample side and a second lens group on an analyzer sideand a deflector device that is configured to deflect the electrons in anexit plane of the electron imaging apparatus in a deflection directionperpendicular to the electron-optical axis, wherein: the first lensgroup provides a first reciprocal plane inside the first lens group anda first Gaussian plane between the first and the second lens group andis configured to generate a first momentum distribution image of amomentum distribution of electrons from the sample in the firstreciprocal plane and to generate a first Gaussian image of the sample inthe first Gaussian plane, the second lens group provides a secondreciprocal plane on the analyzer side of the second lens group and isconfigured to generate a second momentum distribution image of themomentum distribution of the electrons from the sample in the secondreciprocal plane, and the first lens group is configured to generate thefirst Gaussian image in a front focal plane of the second lens groupwith such a small dimension that the second momentum distribution imagegenerated by the second lens group is a parallel image.
 2. The electronimaging apparatus according to claim 1, wherein the deflector device isconfigured such that the deflector device acts in one single planeperpendicular to the optical axis.
 3. The electron imaging apparatusaccording to claim 1, wherein the deflector device comprises one singlepair of at least one of electrically and magnetically acting deflectorelements, a quadrupole arrangement of four deflector elements in oneplane or an octupole arrangement of eight deflector elements in oneplane.
 4. The electron imaging apparatus according to claim 1, wherein:the second reciprocal plane is an exit plane of the electron imagingapparatus, and a slit diaphragm is arranged in the second reciprocalplane, forming an entry slit of the energy analyzer apparatus.
 5. Theelectron imaging apparatus according to claim 1, further comprising: atleast one third lens group, that is arranged on the analyzer side of thesecond lens group and forms a second Gaussian plane inside the at leastone third lens group and a third reciprocal plane on the analyzer sideof the at least one third lens group and is configured to generate asecond Gaussian image of the sample in the second Gaussian plane and athird momentum distribution image of the momentum distribution of theelectrons from the sample in the third reciprocal plane, wherein thethird reciprocal plane is the exit plane of the electron imagingapparatus, and the third momentum distribution image generated by the atleast one third lens group is a second parallel image.
 6. The electronimaging apparatus according to claim 5, wherein a slit diaphragm isarranged in the second reciprocal plane, said slit diaphragm forming anentry slit of the energy analyzer apparatus by imaging into the entryplane of the energy analyzer apparatus.
 7. The electron imagingapparatus according to claim 5, wherein a slit diaphragm is arranged inthe third reciprocal plane, forming an entry slit of the energy analyzerapparatus, wherein no slit diaphragm is arranged in the secondreciprocal plane.
 8. The electron imaging apparatus according to claim1, wherein the deflector device is arranged between the first and thesecond lens group and in the first Gaussian plane.
 9. The electronimaging apparatus according to claim 5, wherein the deflector device isarranged at the third lens group and in the second Gaussian plane. 10.The electron imaging apparatus according to claim 1, wherein the firstlens group is configured to generate the first Gaussian image with anextent perpendicular to the electron-optical axis of less than 1 mm. 11.The electron imaging apparatus according to claim 1, wherein the firstand the second lens group are configured to form the parallel image withangular deviations of its partial beams of less than 0.4°.
 12. Theelectron imaging apparatus according to claim 1, wherein a foremostsample-side electron-optical element of the first lens group isconfigured to have a same potential as the sample, so as to generate afield-free area in front of the sample.
 13. The electron imagingapparatus according to claim 1, wherein the deflector device is coupledto a control device, which is configured for a scanning deflection ofthe electrons in the exit plane of the electron imaging apparatus whilepreserving the parallel image.
 14. An electron spectrometer apparatus,comprising: a sample-holder configured to hold a sample, an electronimaging apparatus according to claim 1, and an energy analyzerapparatus, wherein the electron imaging apparatus is configured forelectron transfer of electrons emitted from the sample along theelectron-optical axis to the energy analyzer apparatus.
 15. The electronspectrometer apparatus according to claim 14, wherein the energyanalyzer apparatus comprises a hemispherical analyzer.
 16. An electrontransfer method, wherein electrons from a sample are transferred by anelectron imaging apparatus along an electron-optical axis to an energyanalyzer apparatus, wherein the electrons pass in sequence through asample-side first lens group and an analyzer-side second lens group andthe electrons are deflected by a deflector device in an exit plane ofthe electron imaging apparatus in a deflection direction perpendicularto the electron-optical axis, wherein: the first lens group forms afirst reciprocal plane inside the first lens group and a first Gaussianplane between the first and the second lens group and generates a firstmomentum distribution image of a momentum distribution of electrons fromthe sample in the first reciprocal plane and a first Gaussian image ofthe sample in the first Gaussian plane, the second lens group forms asecond reciprocal plane on the analyzer side of the second lens groupand generates a second momentum distribution image of the momentumdistribution of the electrons from the sample in the second reciprocalplane, and the first lens group generates the first Gaussian image in afront focal plane of the second lens group with such a small dimensionthat the second momentum distribution image generated by the second lensgroup is a parallel image.
 17. The electron transfer method according toclaim 16, wherein the deflector device is configured such that thedeflector device acts in one single plane perpendicular to the opticalaxis.
 18. The electron transfer method according to claim 16, wherein:the second reciprocal plane is the exit plane of the electron imagingapparatus, and a slit diaphragm that forms an entrance slit of theenergy analyzer apparatus is arranged in the second reciprocal plane.19. The electron transfer method according to claim 16, wherein at leastone third lens group that is arranged on the analyzer side of the secondlens group forms a second Gaussian plane inside the at least one thirdlens group and a third reciprocal plane on the analyzer side of the atleast one third lens group and generates a second Gaussian image of thesample in the second Gaussian plane and a third momentum distributionimage of the momentum distribution of the electrons from the sample inthe third reciprocal plane, wherein: the third reciprocal plane is theexit plane of the electron imaging apparatus, and the third momentumdistribution image generated by the at least one third lens group is asecond parallel image.
 20. The electron transfer method according toclaim 19, wherein a slit diaphragm is arranged in the second reciprocalplane, said slit diaphragm forming an entry slit of the energy analyzerapparatus by imaging into the entry plane of the energy analyzerapparatus.
 21. The electron transfer method according to claim 19,wherein a slit diaphragm is arranged in the third reciprocal plane,forming an entry slit of the energy analyzer apparatus, wherein no slitdiaphragm is arranged in the second reciprocal plane.
 22. The electrontransfer method according to claim 16, wherein the deflector device isarranged in the first Gaussian plane.
 23. The electron imaging apparatusaccording to claim 19, wherein the deflector device is arranged at theat least one third lens group and in the second Gaussian plane.
 24. Theelectron transfer method according to claim 16, wherein the first lensgroup generates the first Gaussian image with an extent perpendicular tothe electron-optical axis of less than 1 mm.
 25. The electron transfermethod according to claim 16, wherein the first and the second lensgroup are configured to form the parallel image with angular deviationsof its partial beams of less than 0.4°.
 26. The electron transfer methodaccording to claim 16, wherein a foremost sample-side electron-opticalelement (11) of the first lens group has the same potential like thesample, so as to generate a field-free area in the region of the sample.27. The electron transfer method according to claim 16, wherein theelectrons in the exit plane of the electron imaging apparatus aredeflected to generate a scanning motion of the momentum distributionimage while preserving the parallel image.
 28. An electron spectroscopymethod, comprising the steps: irradiation of a sample and emission ofelectrons from the sample, transfer of the electrons emitted from thesample by an electron transfer method according to claim 16 to an energyanalyzer apparatus, and energy-resolved detection of electrons by theenergy analyzer apparatus.
 29. The electron spectrometry methodaccording to claim 28, wherein the energy analyzer apparatus comprises ahemispherical analyzer.